1. Field of the Invention
The present invention relates to multilayer structures, and more particularly, to multilayer structures having annular profiles and methods and apparatus of making the same.
2. Discussion of the Related Art
The current multilayer film processing technologies are referred to as cast film and blown films. Cast film processes use a flat planar type of production process and are suited to produce flat plastic film and sheet that often have up to about 15% edge trim. Blown film process are known to provide greater flexibility in film or sheet width changes on the same line, achieve better economics in lower volume specialty applications where frequent product changeover is required and typically avoid the yield losses associated with edge trim.
Multilayer films are made by known layering processes typically using a uni-axial cast or planar sheet process or lamination. Coextruded cast film or sheet structures typically have 3 to 5 layers; however, cast film or sheet structures including hundreds of layers are known. For example, early multilayer processes and structures are shown in U.S. Pat. Nos. 3,565,985; 3,557,265; and 3,884,606. WO 2008/008875 discloses a related art method of forming multi-layered structures having many, for example fifty to several hundred, alternating layers of foam and film. The processes as shown, however, induce only substantially uni-axial orientation, namely, in the machine direction. This is disadvantageous since the resulting structures may possess unbalanced mechanical properties due to very unbalanced orientation. Subsequent orientation processes can be used (e.g. tenter-frame process) to achieve bi-axial orientation. These additional processes are elaborate and expensive and the desired degrees of orientation may be different than desired because it occurs with dimensional limitations and at a relatively cooler polymer temperature below the melting point of the highest melting point polymer in the multilayer film.
Multilayer structures having annular profiles with limited numbers of layers are used in numerous applications. These annularly shaped, tube-like structures include, for example, the “bubbles” in blown film processes, coatings on wires or cables, blow molded articles and the parisons or preforms used in their production, and pipe. Such articles typically contain 2 to 10 layers and have annular layers supplied by separate manifolds. Orientation process steps in the extrusion of annular profiles and products, such as the inflation of a blow molded article or the “bubble” in a blown film process, can very advantageously be utilized to provide biaxial orientation (sometime referred to as multi-axial orientation) that is known to provide polymer resin articles with very advantageous combinations of physical properties.
As well known in the art, blown film, blow molded and other annular shaped articles may be made by feeding a polymer melt flow into a distribution manifold of an annular die. Obtaining multiple layers generally requires a distribution manifold or mandrel to be designed and fabricated for each layer; e.g. a 6 layer annular structure would be made using a die containing 6 individual distribution manifolds, one for each layer. The design and fabrication of these multiple distribution manifolds to produce annular structures with a large numbers of layers is very difficult and limited in the number of annular layers that can be produced in a structure. See for example a sequential manifold layering technique for an annular die, as taught in Dooley, J. and Tung, H., Co-extrusion, Encyclopedia of Polymer Science and Technology, John Wiley & Sons, Inc., New York (2002).
Another method of making a multilayered structure having an annular profile includes using a spiral mandrel die. In a spiral mandrel/distribution manifold die, the polymer melt flow fed to the distribution manifold of the die flows through a manifold channel which is spirally cut from the entry to near the exit of the manifold, as described in Extrusion Dies, Design and Engineering Computations, Walter Michaeli, 1984, pages 146-147. The flow through the distribution manifold of the spiral die is not suitable for processing more than a single layer melt flow in a single distribution manifold since it would cause a multi-layered melt flow to become discontinuous and lose layer integrity.
U.S. Pat. Nos. 3,308,508, 5,762,971 and 6,413,595 disclose forming an annular multilayer structure in a so-called pancake die (also known as planar geometry). The pancake die includes multiple stacked planar or flat distribution manifolds. Each of several polymer melt flows is fed into a distribution manifold. The multilayered structure is formed by joining the several concentric melt flows after each melt flow exits its distribution manifold. If a large number of layers are desired, a large number of stacked manifolds are required. This can lead to a large pressure drop and extended residence times in the die. U.S. Pat. Nos. 5,762,971 and 6,413,595 disclose producing a final multilayer structure having a maximum of about 27 layers.
Using a spiral pancake die, multi-layered structures having up to 11 layers are known. However, these multi-layered structures are similarly made by stacking several spiral distribution manifolds on each other to form one annular die and combining the melt flow streams as they are exiting the entire annular die.
Another related art method of making a multilayered structure having an annular profile includes using an annular die, such as that described in U.S. Pat. No. 6,685,872. As disclosed, 3 layers are fed into one single distribution manifold of the annular die. The disclosed manifold design provides an annular multilayer structure which has a non-uniform circumference with a designed overlap section where the layer structure is overlapped in such a way that the overlapped area at least maintains the barrier properties of the layer structure in the non-overlapped area.
US 2008/0157443 describes a method and apparatus for making a parison. The apparatus has a mandrel housing with a side channel substantially transverse to the mandrel channel. The mandrel has an axially oriented notch in an exterior surface which is in fluid communication with two fluid channels that extend continuously downwardly around the mandrel to meet one another on the opposite side of the mandrel from the notch. The examples disclose structures having up to 17 layers, although it discusses composite streams having up to 100 layers.
However, there is always a need to produce annular multilayered structures having a larger number of layers; use a reduced number of distribution manifolds in a die; produce annular multilayered structures having improved combinations of physical and mechanical properties; and/or reduce the number of processing steps and increase flexibility in annular structure production equipment.